A number of countries have committed to allow cognitive radio (CR) access to unlicensed spectrum, or what is commonly referred to as Television White Space (TVWS). This is to be performed under the authorization of geolocation databases, although how multiple TVWS devices (WSD) should share the available TVWS channels once they are granted is currently neither regulated nor standardized. One possible solution is to allow geolocation databases to co-ordinate the sharing of TVWS among potentially interfering devices based on location information. However, in some territories such as the USA, the regulator has permitted multiple geolocation database providers to exist in order to promote competition. This is a trend that is likely to be followed elsewhere.
A database-centric approach to TVWS sharing may not, therefore, be feasible or efficient as it will require frequent communications between the different database providers in order to share information on TVWS usage by WSDs, which of course will change frequently.
The IEEE 802.19 standard is attempting to address this issue by introducing a centralized spectrum management entity to which WSDs would voluntarily subscribe. This however is likely to suffer from scalability issues due to proliferation of TVWS networks. Furthermore, it is not clear how, without regulatory intervention, WSDs using different access technologies from the IEEE 802 families (such as LTE) would voluntarily subscribe to such a management entity.
Autonomous spectrum sharing protocols (or spectrum etiquettes or politeness protocols) are known whereby devices autonomously regulate the sharing of spectrum amongst themselves. One example is adaptive channel selection algorithms used in IEEE 802.11 (WiFi) access points which operate in the Industrial Scientific and Medical (ISM) bands. These protocols distribute available frequency channels amongst themselves to minimize interference. CSMA-CA is then used to enable time-sharing between potentially interfering devices operating in the same channel.
Further information on such a conventional protocol is described in “A self-managed distributed channel selection for WLANs” by D. J. Leith, P. Clifford, 4th IEEE International Symposium on Modelling and Optimization in Mobile, Adhoc and Wireless Networks, pages 1-9, 2006.
Devices operating according to these protocols in the ISM band generally operate at low transmit powers (up to 100 mW in the UK and Europe and 1 W in the United States) and use the same air-interface standard. WSDs, on the other hand, will likely use different transmit powers. For example, devices using the emerging IEEE 802.11af standard will typically use 100 mW (EIRP) or lower, whilst devices using the IEEE 802.22 standard (or LTE or WiMAX) for rural broadband access may use up to 4 W (EIRP). Furthermore, low power devices generally use omnidirectional antennas whereas high power devices tend to use directional antennas. Referring to FIG. 1, modelling of the conventional WiFi adaptive channel selection algorithms suggests that, as more high power devices enter a wireless network, the average interference actually increases. Even introducing a small number of high power devices has a detrimental effect. The same problem is found to occur if, instead of using different transmit powers, some WSDs use directional antennas whilst others use omnidirectional antennas.
The reason for this effect is likely due to the asymmetrical nature of interference between low and high power devices (or devices having directional and omnidirectional antennas).
Furthermore, WSDs will likely use different air interface standards (WiFi, LTE, MiMAX, Weighless). Conventional WiFi adaptive channel selection and CSMA-CA methods were not developed to operate in the heterogeneous network scenarios envisaged for TVWS.